As disclosed in US 2005/0269879 A1, recently, vehicles have been equipped with an airbag apparatus to protect a vehicle occupant. The airbag apparatus inflates an airbag when a vehicle crashes. The airbag apparatus disclosed in US 2005/0269879 A1 includes a safing switching element (safing switch). The safing switch regulates a voltage supplied from a power source, such as a battery, to a target voltage, and supplies the regulated voltage to a circuit (activation circuit) that provides a current to fire a squib. Hereinafter, the voltage supplied from the power source is also referred to as a supply voltage. The activation circuit includes the squib, and a high side switching element (high side switch) that regulates a current flowing through the squib. The high side switch has an input terminal connected with an output terminal of the safing switch, and an output terminal connected with the squib. Usually, the airbag apparatus includes multiple above-described activation circuits, and the output terminal of the safing switch is connected with the input terminal of the high side switch of each activation circuit.
In the above-described airbag apparatus, a short circuit may occur in a wire connecting the high side switch with the squib (squib line), causing the supply voltage from the battery to be directly applied to the squib line. When the short circuit occurs, the target voltage becomes lower than the supply voltage supplied by the battery. The target voltage, which is lower than the supply voltage, may cause the following problems. In one of the activation circuits, when the short circuit occurs, the output terminal of the high side switch has a voltage higher than the input terminal of the high side switch. Thus, the current may flow from the input terminal of the high side switch to another activation circuit, and a reverse current is generated and flows through the high side switch. In this state, when the high side switch of another activation circuit is turned on, the current flows from the battery to another activation circuit via the high side switch of the short-circuited activation circuit. This reverse current may concentrate on the high side switch of the short-circuited activation circuit, causing it to break. If the high side switch of the short-circuited activation circuit breaks, an integrated circuit including the high side switch may also break, and proper operation of the other activation circuits may be affected.
In order to prevent breakage of the high side switch of the short-circuited activation circuit due to the reverse current, the target voltage is set higher than a maximum supply voltage of the battery.
When the target voltage is set higher than the maximum supply voltage of the battery, the high side switch used in the activation circuit is required to allow the target voltage to be higher than the maximum supply voltage of the battery. As is well known, a size of the high side switch increases with an increase in an allowable voltage of the high side switch. Thus, when target voltage is set higher than the maximum supply voltage of the battery, the size of the high side switch of the activation circuit increases. Accordingly, a size of the activation circuit increases.